API: Add endpoint to delete a group share

What does this MR do?

Add the ability to unshare from a group.

Are there points in the code the reviewer needs to double check?

To be able to retrieve the link id, I expose it for the SharedGroup. It is named id for now. Shall we change that to link_id to be consistent with the delete endpoint?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24771 (closed)